The system that has been frequently used in recent years in sensing devices in entry warning systems is such that two pyroelectric sensors made of ferroelectric substance are horizontally arranged and their output terminals are connected in series or parallel to provide a differential output. Since two detection axes for two sensors (usually the incident axes of concave mirrors for causing extreme infrared rays from an object to be focused on these sensors, or incident axis from the light source to the sensors when near infrared rays or other visible light rays are used) are spaced tens of centimeters at a maximum from each other along a direction in which objects to be detected come in, when a person or an object moves in that direction or in the opposite direction across the detection axes, signals of opposite polarities will be successively produced. However, when a falling object crosses the two detection axes at the same time, the resulting two sensor outputs cancel each other, so that the resultant signal is zero. Thus, in the twin system, a warning can be produced when a person or the like successively crosses term at the level of disposition (detection level) of the two sensors.
The twin system is capable of detecting the direction of movement in a horizontal plane in this manner, as compared with the single eye system, but it has the disadvantages that it cannot judge the height of an entering object and that detection with mere two points (two eyes) is subject to the influence of a disturbance, that is, in twin sensors, if an insect or a small animal or some object moves in the direction of disposition of the sensors at their level of disposition (or detection level), a warning will be issued (erroneous warning). Although detection with only two points is not influenced by substantially uniform air cooling which covers these points, there is a danger that an erroneous warning is produced in response to the cooling or heating of only one point by a draft or direct sunlight, or that reversely an entering person or the like is overlooked.